Silica gel

ABSTRACT

A method of producing a silica gel by hydrolyzing a silicon alkoxide and subjecting the resulting hydrogel to a hydrothermal treatment substantially without aging it is described. Also described in a silica gel produced by such a method and a silica gel which has the following characteristics:
         (a) the pore volume is from 0.6 to 1.6 ml/g,   (b) the specific surface area is from 300 to 900 m 2 /g,   (c) the mode diameter (Dmax) of pores is less than 20 nm,   (d) the volume of pores having diameters within ±20% of Dmax is at least 50% of the total pore volume,   (e) it is amorphous, and   (f) the content of metal impurities is at most 500 ppm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel silica gel which is excellentin e.g. heat resistance and hydrothermal resistance.

2. Discussion of Background

Silica gel has been widely used as a drying agent and in recent years,it has been used for various purposes as e.g. a catalyst support, aseparating medium or an adsorbent. Accordingly, requirements forperformances of the silica gel have been diversified according to thepurposes. The performances of the silica gel are greatly influenced byphysical properties of the silica gel such as the surface area, the porediameter, the pore volume, the pore size distribution, and thesephysical properties are greatly influenced by conditions for producingthe silica gel.

As a method for producing the silica gel, the most common method is tohydrolyze an alkali silicate such as sodium silicate with a mineralacid, and gelate the resulting silica hydrosol, followed by drying. Manyproposals have been made with respect to the details of the productionmethod so as to improve the performances of the silica gel.

For example, JP-A-62-113713 proposes a method for producing a silica gelhaving a narrow pore distribution, produced by gelling silica hydrosolformed by reaction of alkali silicate aqueous solution and mineral acidsolution, treating a silica hydrogel with acid solution at a pH under2.5, washing with water, adjusting the pH to 4-9 in a buffer solutionand treating hydrothermally. In Examples of the above gazette, a silicagel having an average pore diameter of from 6.7 to 8.5 nm and a porevolume of from about 0.8 to about 0.9 ml/g can be obtained by the abovemethod.

Further, JP-A-9-30809 proposes a method wherein a silica hydrogel isdried by batch flow drying and then a hydrothermal treatment is carriedout. Changes in performances of the silica gel obtained by this methodare also confirmed, and a silica gel having a sharper pore distributioncan be obtained. However, the pore volume, the specific surface area andthe average pore diameter can not adequately be changed, and this methodis inadequate as a method to obtain a silica gel having desired physicalproperties.

On the other hand, in the silica gel obtained by using an alkalisilicate as a material as explained above, usually a considerable amountof impurities such as sodium, calcium, magnesium, titanium, aluminum andzirconium derived from the material is contained. The metal impuritiesin the silica gel can have a significant influence over the performancesof the silica gel even if the total content is so small as at a level ofseveral hundreds ppm. For example, metal impurities can (1) acceleratecrystallization of the silica gel at a high temperature, 2) accelerate ahydrothermal reaction of the silica gel in the presence of water tocause an increase in the pore diameter and the pore volume, a decreasein the specific surface area and a broadening in the pore distribution,and 3) decrease the sintering temperature, and accordingly when a silicagel containing them is heated, cause a decrease in the specific surfacearea tends to be accelerated. These influences tend to be significantwith impurities of alkali metals and alkaline earth metals. Further, iftitanium or aluminum as an impurity are present on the surface of thesilica gel or in a siloxane bond, the acidification site tends toincrease, and the silica gel itself may show an unfavorable catalyticeffect when used as a catalyst carrier or an adsorbent.

Kim et al. (Ultrastable Mesostructured Silica Vesicles Science, 282,1302 (1998) describes the preparation of mesoporous molecular sieves ofhigh thermal (1000° C.) and hydrothermal stabilities (more than 150hours at 100° C.) by a supramolecular assembly pathway which relies onhydrogen bonding between electrically neutral gemini surfactants andsilica precursors derived from tetraethylorthosilicates.

Accordingly, as a method for producing silica gel containing noimpurities, a method of purifying a gel obtained by neutralizing analkali silicate and a method of hydrolyzing a silicon alkoxide have beenknown, and particularly by the latter method, the silicone alkoxide canbe purified by e.g. distillation, and it is thereby possible to obtain asilica gel having a high purity relatively easily. However, a silica gelobtained by a sol-gel method from a silicon alkoxide tends to have asmall average pore diameter and a broad pore distribution in general.Further, substantially no report has been made with respect to animprovement in performances even if a hydrothermal treatment is appliedto the silica gel.

SUMMARY OF THE INVENTION

Under these circumstances, it is an object of the present invention toprovide a method of producing a silica gel having not only large porevolume and specific surface area, but also a narrow pore distributionand excellent heat resistance and hydrothermal resistance.

According to another embodiment of the present invention is a silica gelproduced by a method of producing a silica gel having not only largepore volume and specific surface area, but also a narrow poredistribution and excellent heat resistance and hydrothermal resistance.

According to another embodiment of the present invention is a silica gelhaving not only large pore volume and specific surface area, but also anarrow pore distribution and excellent heat resistance and hydrothermalresistance.

These and other object of the present invention are made possible by amethod of producing a silica gel by hydrolyzing a silicon alkoxide andsubjecting the resulting hydrogel to a hydrothermal treatmentsubstantially without aging it.

The silica gel according to the present invention may be characterizedas having the following characteristics:

-   -   (a) the pore volume is from 0.6 to 1.6 ml/g,    -   (b) the specific surface area is from 300 to 900 m²/g,    -   (c) the mode diameter (Dmax) of pores is less than 20 nm,    -   (d) the volume of pores having diameters within ±20% of Dmax is        at least 50% of the total pore volume,    -   (e) it is amorphous, and    -   (f) the content of metal impurities is at most 500 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same become betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating the relation between the aging time andthe breaking stress of a hydrogel of silica when subjected to aging attemperatures of 35° C., 45° C. and 55° C.

FIG. 2 is a diagram illustrating pore diameter distribution of silicagels of Examples of the present invention.

FIG. 3 is a diagram illustrating a powder X-ray diffraction spectrum (atthe low-angle side) of the silica gels of Examples of the presentinvention.

FIG. 4 is a diagram illustrating a powder X-ray diffraction spectrum (atthe wide-angle side) of the silica gels of Examples of the presentinvention.

FIG. 5 is a diagram illustrating pore diameter distribution ofcommercially available silica gels in Comparative Examples.

FIG. 6 is a diagram illustrating changes in the specific surface areasby heat treatment of silica gels of Examples of the present inventionand silica gels of Comparative Examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail below.

The silica gel of the present invention has a pore volume and a specificsurface area larger than those of conventional one, and the pore volumeis generally from 0.6 to 1.6 ml/g, preferably from 0.8 to 1.6 ml/g asmeasured by a nitrogen gas adsorption/desorption method. Further, thespecific surface area is generally from 300 to 900 m²/g, preferably from400 to 900 m²/g. The pore volume and the specific surface area can bemeasured by a BET method, by nitrogen gas adsorption/desorption.

Further, the silica gel of the present invention has a mode diameter(Dmax) of less than 20 nm on a pore distribution curve calculated by aBJH method as disclosed in E. P. Barrett, L. G. Joyner, P. H. Haklenda,J. Amer. Chem. Soc., vol. 73, 373 (1951) from an isothermal desorptioncurve measured by a nitrogen gas adsorption/desorption method, i.e. afigure obtained by plotting a differential nitrogen gas adsorptionamount (ΔV/Δ (logd): V is the nitrogen gas adsorption volume) relativeto the pore diameter d (nm). The lower limit is not particularlylimited, but it is preferably at least 2 nm. This means that the modediameter (Dmax) of the silica gel of the present invention is smallerthan that of a conventional silica gel.

The silica gel of the present invention is characterized by that thevolume of pores having diameters within ±20% of the above mode diameter(Dmax) is at least 50%, more preferably at least 60%, of the total porevolume. Typically the volume of pores having diameters within ±20% ofthe above mode diameter (Dmax) is at most 90%. This means that thesilica gel of the present invention comprises pores having diameters inthe vicinity of the mode diameter (Dmax).

Further, the silica gel of the present invention has a differential porevolume ΔV/Δ (logd) particularly preferably of from 5.0 to 12.0 ml/g atthe mode diameter (Dmax) as calculated by the above BJH method (here, inthe above formula, d is the pore diameter (nm) and V is the nitrogen gasadsorption volume). The silica gel of the present invention wherein thedifferential pore volume ΔV/Δ (logd) is within the above range has anextremely sharp peak of the mode diameter (Dmax).

The silica gel of the present invention is characterized, in addition tothe above characteristics in the pore structure, by being amorphous inview of its three-dimensional structure, namely, it is characterized inthat no crystalline structure can be confirmed therein. This means thatsubstantially no crystalline peak can be confirmed when the silica gelof the present invention is analyzed by X-ray diffraction. Within thecontext of the present invention a silica gel will not be amorphous ifit has an X-ray diffraction pattern with at least one peak indicatingcrystalline structure at a position greater than 6 Å units d-spacing.The silica gel which is amorphous has an excellent hydrothermalresistance as compared with the silica gel which is crystalline.

Further, with respect to the structure of the silica gel of the presentinvention, characteristic results can be obtained in analysis by solidstate Si-NMR. Namely, in solid state Si-NMR, the value of “Q4/Q3” whichindicates the molar ratio of Si having four —OSi bonded thereto to Sihaving three —OSi bonded thereto of the silica gel of the presentinvention is usually at least 1.3, preferably at least 1.5. It isconsidered that heat stability is high when the value of “Q4/Q3” is highin general. Typically the value of “Q4/Q3” will be no greater than 10.

The last characteristic of the silica gel of the present invention isthat the total content of metal impurities except for siliconconstituting the skeleton of the silica gel is at most 500 ppm,preferably at most 100 ppm, more preferably at most 10 ppm, mostpreferably at most 1 ppm, and the silica gel has an extremely highpurity. Such a small influence of the impurities is one of great factorsto show excellent properties such as heat resistance and hydrothermalresistance of the silica gel of the present invention.

A method for producing the silica gel of the present invention havingphysical properties as explained above is not particularly limited otherthan by not aging prior to hydrothermal treatment, and the silica gelmay be produced by applying a method of subjecting to a hydrothermaltreatment, a silica hydrogel obtained by hydrolyzing an alkali silicateor obtained by hydrolyzing a silicon alkoxide, preferably by hydrolyzinga silicon alkoxide.

The silicon alkoxide to be used as a material for the silica gel of thepresent invention may, for example, be a tri- or tetraalkoxysilanehaving a C₁₋₄ lower alkyl group or its oligomer, such astrimethoxysilane, tetramethoxysilane, triethoxysilane,tetraethoxysilane, tetrapropoxysilane or tetrabutoxysilane, andpreferred are tetramethoxysilane, tetraethoxysilane and their oligomers.The above silicon alkoxides may be easily purified by distillation andare suitable as a material for a silica gel having a high purity. Thetotal content of metal impurities in the silicon alkoxides is preferablyat most 100 ppm, more preferably at most 10 ppm. The content of metalimpurities may be measured by the same method as the one used todetermine impurities in the silica gel.

The hydrolysis of the silicon alkoxide may be carried out by usingusually from 2 to 10 mol, preferably from 3 to 8 mol of water based on 1mol of the silicon alkoxide. A hydrogel of silica and an alcohol areformed by the hydrolysis of the silicon alkoxide. This hydrolysis may becarried out usually at a temperature of from room temperature to about100° C., but may be carried out at a high temperature by maintaining theliquid phase under elevated pressure. The reaction time depends on thereaction solution composition (type of the silicon alkoxide and themolar ratio with water) and the reaction temperature, and is notnecessarily defined since the time for gelation can vary. Preferably,the reaction time is at most the time that a breaking stress of ahydrogel becomes at most 6 MPa. Here, an acid, an alkali, a salt or thelike may be added to the reaction system as a catalyst to accelerate thehydrolysis. However, use of such an additive may cause aging of theformed hydrogel as described hereinafter, and is thereby unfavorable forproduction of the silica gel of the present invention.

In the above hydrolysis reaction of the silicon alkoxide, a siliconalkoxide undergoes hydrolysis so that the silicate is produced andsubsequently a condensation reaction of the silicate takes place, theviscosity of the reaction solution increases, and the silicate undergoesgelation to form a hydrogel and is solidified finally. To produce thesilica gel of the present invention, it is important to immediatelycarry out a hydrothermal treatment without substantially aging thehydrogel of silica formed by the hydrolysis so that it is not completelysolidified. When the silicon alkoxide is hydrolyzed, a weak hydrogel ofsilica is formed, and by a conventional method of subjecting thishydrogel to aging, drying and a hydrothermal treatment to finally obtaina stably hard silica gel, no silica gel having physical propertieswithin ranges as specified in the present invention can be produced.

To immediately subject the hydrogel of silica formed by the hydrolysisto a hydrothermal treatment substantially without aging as mentionedabove, means that the hydrogel of silica is subjected to a subsequenthydrothermal treatment while maintaining a weak state immediately afterthe hydrogel of silica is formed. It is not preferred to add e.g. anacid, an alkali or a salt to the hydrolysis reaction system of thesilicon alkoxide or to raise the temperature of the hydrolysis reactiontoo strictly, which accelerates aging of the hydrogel. Further, thetemperature should not be raised too high or the time should not betaken so long more than necessary when the hydrogel is washed withwater, dried or left to stand as an after-treatment after thehydrolysis.

As a means to specifically confirm the aging state of the hydrogel, thehardness of the hydrogel measured by a method as disclosed in Examplesdescribed hereinafter may be employed. Namely, a silica gel havingphysical properties within ranges as specified in the present inventioncan be obtained by subjecting a hydrogel in a soft state having abreaking stress of usually at most 6 MPa, preferably at most 3 MPa, morepreferably at most 2 MPa, to a hydrothermal treatment.

As conditions of the hydrothermal treatment, water may be in eitherstate of liquid or gas, and may be diluted by a solvent or another gas,but water in a liquid state is preferably used. Water in an amount ofusually from 0.1 to 10 times, preferably from 0.5 to 5 times,particularly preferably from 1 to 3 times of the weight of the hydrogelof silica is added to obtain a slurry, and the treatment is carried outat a temperature of usually from 40 to 250° C., preferably from 50 to150° C., for usually from 0.1 to 100 hours, preferably from 1 to 10hours. In the water to be used for the hydrothermal treatment, e.g. alower alcohol, methanol, ethanol or propanol may be contained. Thishydrothermal treatment method can be applied to a case of a materialhaving silica gel in a form of a membrane or layers formed on a matrixsuch as particles, a substrate or a tube, with a purpose of making amembrane reactor. Here, it is possible to carry out the hydrothermaltreatment by using a reactor for the hydrolysis reaction andsubsequently changing the conditions, however, the optimum conditionsare usually different between the hydrolysis reaction and the subsequenthydrothermal treatment, and accordingly it is usually difficult toobtain the silica gel of the present invention by such a means.

With respect to the above hydrothermal treatment conditions, when thetemperature is raised, the pore diameter and the pore volume of theobtained silica gel tend to be large. Further, the specific surface areaof the silica gel tends reach to the maximum once and then graduallydecrease along with the treatment time. Accordingly, it is necessary toselect proper conditions depending upon the desired physical propertiestaking the above tendencies into consideration. However, the physicalproperties of the silica gel are likely to change in the hydrothermaltreatment, and accordingly it is preferred to carry out the hydrothermaltreatment at a temperature higher than the above hydrolysis reactioncondition in general.

It tends to be difficult to obtain the silica gel of the presentinvention if the temperature and the time for the hydrothermal treatmentare beyond the above ranges. For example, if the temperature of thehydrothermal treatment is too high, the pore diameter and the porevolume of the silica gel tend to be too large, and the pore distributiontends to be broad. On the contrary, if the temperature of thehydrothermal treatments is too low, the formed silica gel tends to havea low degree of crosslinking and tends to be poor in heat stability, andno peak may appear in the pore distribution, or the above Q4/Q3 valuetends to be extremely small in the solid state Si-NMR.

Here, when the hydrothermal treatment is carried out in ammonia water,the same effects can be obtained at a temperature lower than that in acase of carrying out the treatment in pure water. Further, when thehydrothermal treatment is carried out in ammonia water, the silica gelto be obtained finally tends to be hydrophobic in general as comparedwith a case of the treatment in pure water, and the hydrophobicitybecomes particularly high when the hydrothermal treatment is carried outat a relatively high temperature of from 100 to 150° C. Here, theammonia concentration of the ammonia water is preferably from 0.001 to10%, particularly preferably from 0.005 to 5%.

The silica hydrogel subjected to the hydrothermal treatment is driedusually at from 40 to 200° C., preferably from 60 to 120° C. The dryingmethod is not particularly limited, and it may be a batch or continuoussystem, and may be carried out under normal or reduced pressure. In acase where a carbon content derived from the silicon alkoxide as thematerial is contained, it may be removed by sintering at a temperatureof usually from 400 to 600° C., as the case requires. The silicahydrogel may be heated to remove the carbon content derived from thesilicon alkoxide as the material. Further, pulverization andclassification may be carried out as the case requires to obtain thefinal desired silica gel of the present invention.

The silica gel which has a crystalline structure tends to be poor inhydrothermal resistance and the gel easily includes crystallinestructure when hydrolysis of silicon alkoxide proceeds in the presenceof a template such as a surfactant, used for the formation of pores inthe gel. Accordingly, in a preferred embodiment of the presentinvention, hydrolysis is conducted in the absence of a template, such asa surfactant in an amount sufficient to function as a template.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted to such specific Examples.

(1) Measurement of Hardness of Hydrogel of Silica

A silicon alkoxide and water in an amount of 6 molar times the siliconalkoxide were reacted in a 5 liter separable flask, and after thetemperature of the reaction solution reached the boiling point of thealcohol to be formed by the reaction, the reaction solution was drawnfrom the flask, and the drawn reaction solution was put in a 50 cc glassscrew tube in a certain amount (at a level of 20 mm as the liquid depth)and sealed, and the tube was held in a water bath having its temperaturecontrolled to be substantially constant, and the disruptive strength wasmeasured by means of a digital force gage (manufactured by A & D, model:AD-4935) with the passage of aging time. A probe (stainless round barhaving a diameter of 5 mm) was attached to the measuring apparatus, andwhen it was slowly pushed into the hydrogel, it broke the hydrogel heldin the container by compression. The maximum stress applied until thehydrogel was broken by compression was taken as the breaking stress.

The measurement results are shown in FIG. 1. FIG. 1 was obtained byplotting the common logarithm of the aging time of the hydrogel ofsilica on the horizontal axis and the breaking stress on the verticalaxis. It is found from FIG. 1 that the breaking stress becomes highalong with the passage of aging time and that the aging rate depends onthe temperature.

(2) Analysis Method of Silica Gel

1) Pore Volume and Specific Surface Area

The pore volume and the specific surface area were obtained by measuringa BET nitrogen adsorption isotherm by AS-1 manufactured by Quantachrome.Specifically, as the pore volume, the value under a relative pressure ofP/P0=0.98 was employed, and the specific surface area was calculatedfrom the nitrogen adsorption amounts at three points under P/P0=0.1, 0.2and 0.3 by a BET multiple method. Further, a pore distribution curve anda differential pore volume at the mode diameter (Dmax) were obtained byBJH method.

2) Powder X-ray Diffraction

Measurement was carried out by means of RAD-RB apparatus manufactured byRigaku Electric Corporation employing CuKα as a source. The divergentslit was ½ deg, the scattering slit was ½ deg, and the acceptance slitwas 0.15 mm.

3) Content of Metal Impurities

Hydrofluoric acid was added to 2.5 g of a sample, followed by heatingand evaporation to dryness, and water was added thereto to obtain 50 mlof an aqueous solution. Using this aqueous solution, ICP emissionspectral analysis was carried out. Here, sodium and potassium wereanalyzed by flame analysis.

4) Solid State Si-NMR (Q4/Q3 Value)

Measurement was carried out by means of MSL300 solid state NMR apparatusmanufactured by Bruker at a resonance frequency of 59.2 MHz (7.05 T)using a 7 mm CP/MAS (Cross Polarization/Magic Angle Spinning) probe.

(2) Production and Evaluation of Silica Gel

EXAMPLES 1 TO 3

1,000 g of pure water was introduced into a 5 liter separable flask(jacketed) made of glass and equipped with a water-cooled condenser ofair open system at the upper part thereof. 1,400 g of tetramethoxysilanewas introduced thereto over a period of 3 minutes with stirring at 80rpm. The molar ratio of water/tetramethoxysilane was about 6. Hot waterof 50° C. was passed through the jacket of the separable flask. Stirringwas subsequently continued, and the stirring was terminated when thetemperature of the content reached the boiling point. The formed sol wasgelated while subsequently passing hot water of 50° C. through thejacket for about 0.5 hour. Then, the gel was immediately taken out andpulverized by means of a nylon net having an opening of 600 μm to obtaina wet gel (silica hydrogel) in a powder state. 450 g of this hydrogeland 450 g of pure water were introduced in a 1 liter glass autoclave,and a hydrothermal treatment was carried out under conditions asidentified in Table 1. After the hydrothermal treatment for apredetermined time, filtration was carried out by means of a No. 5Afilter paper, and the filter cake thus obtained was dried under reducedpressure at 100° C. until a constant weight was reached, without washingthe filter cake with water.

Physical properties of the obtained silica gel are shown in Table 1. Thepore diameter distribution is shown in FIG. 2, the powder X-raydiffraction spectrum at the low-angle side is shown in FIG. 3, and thespectrum at the wide-angle side is shown in FIG. 4. In the powder X-raydiffraction spectra, no crystalline peak appeared, and no peak at thelow-angle side due to a periodic structure (2θ≦5 deg) was confirmed.

Here, with respect to the concentrations of impurities in the obtainedsilica gel, sodium was 0.2 ppm, potassium was 0.1 ppm and calcium was0.2 ppm, and no magnesium, aluminum, titanium and zirconium weredetected, in each of Examples 1 to 3.

EXAMPLE 4

A silica hydrogel was produced in the same manner as in Example 1. 450 gof the silica gel and 450 g of 1 wt % ammonia water were added to a 1liter autoclave, and a hydrothermal treatment was carried out at 60° C.for 3 hours without stirring. Physical properties of the silica gelafter drying are shown in Table 1, and the pore diameter distribution isshown in FIG. 2.

COMPARATIVE EXAMPLE 1

The silica hydrogel produced in Example 1 was put in a sealed containerand left to stand at a cold and dark place (from 10 to 15° C.) for 2weeks for aging, and then a hydrothermal treatment was carried out at60° C. for 3 hours in the same manner as in Example 1. Physicalproperties of the silica gel after drying are shown in Table 1.

COMPARATIVE EXAMPLES 2 TO 4

Physical properties of silica gels for catalyst carrier CARIACT G series(in a pulverized state) manufactured by Fuji Silysia Chemical Ltd. areshown in Table 1, and their pore size distribution are shown in FIG. 5.

Further, the concentrations of metal impurities in G-6 were measured,whereupon sodium was 170 ppm, magnesium was 31 ppm, aluminum was 15 ppm,potassium was 23 ppm, calcium was 160 ppm, titanium was 260 ppm, andzirconium was 44 ppm.

COMPARATIVE EXAMPLE 5

Part of the silica hydrogel used in Example 1 was dried under vacuum at60° C. for 48 hours and then put in a sealed container, and subjected toa hydrothermal treatment at 150° C. for 3 hours. Physical properties ofthe silica gel after drying are shown in Table 1.

TABLE 1 Pore Specific Volume of pores Differential pore TreatmentTreatment volume surface Dmax having diameters volume at Dmax temp. (°C.) time (hr) (ml/g) area (m²/g) (nm) Dmax ± 20% (ml/g) Q4/Q3 Ex. 1 1303 0.85 859 3.9 61 7.7 1.7 Ex. 2 150 3 1.19 706 6.9 69 10.8 2.3 Ex. 3 2003 1.52 409 13.3 77 10.8 3.6 Ex. 4 60 3 1.05 437 8.2 67 6.1 3.2 Comp. Ex.1 60 3 0.37 662 * * * — Comp. Ex. 2 Name of product: 0.53 885 * * * 1.4G-3 Comp. Ex. 3 Name of product: 0.91 599 5.8 48 2.9 2.6 G-6 Comp. Ex. 4Name of product: 1.20 413 18.9 33 3.2 3.8 G-10 Comp. Ex. 5 150 3 0.40562 3.2 35 3.2 — *: Measurement infeasible since no peak appearedTest on Heat Resistance of Silica Gel

5 g of silica gel samples of Examples 1 and 2 and Comparative Examples 2and 3 were put in quartz beakers respectively, and the temperature wasraised to a predetermined heat treatment temperature at a rate of 200°C./min in the air atmosphere in an electric furnace. The beakers wereheld in the predetermined heat treatment temperature for 1 hour and thenimmediately taken out to room temperature and cooled gradually. Withrespect to the samples thus obtained, the specific surface areas weremeasured by BET method by nitrogen gas adsorption/desorption, and theresults are shown in FIG. 6. It is found from FIG. 6 that the silicagels of Examples had a smaller change in the specific surface area bythe heat treatment as compared with the silica gels of ComparativeExamples.

Test on Heat Stability in Water of Silica Gel

Pure water was added to silica gels of Examples 1 to 3 and ComparativeExamples 2 to 4 to prepare slurries of 40 wt %. About 40 ml of theslurries thus prepared were put and sealed in stainless steel vesselshaving a volume of 60 ml respectively, and soaked in an oil bath at280±1° C. for 3 days. Part of each slurry was taken out from each vesseland subjected to filtration by a 5A filter paper. The obtained filtercakes were dried under vacuum at 100° C. for 5 hours. With respect tothese samples thus obtained, the specific surface area was measured andthe results are shown in Table 2.

Further, with respect to the above samples, a powder X-ray diffractionpattern was measured in the same manner as mentioned above except thatthe divergent slit: 1 deg and scattering slit: 1 deg. With respect tothe silica gels of Examples 1 to 3 and Comparative Example 2, no peakappeared and the pattern remained to be an amorphous patter. On theother hand, with respect to the silica gels of Comparative Examples 3and 4, distinct peaks appeared at 2θ±20.9° and 26.6° in the amorphouspattern. These peaks corresponded to peaks of α-quartz, and accordinglyit is considered that part of silica gels of Comparative Examples 3 and4 were crystallized. It is considered that these silica gels underwentcrystallization under hydrothermal conditions of high temperature andhigh pressure and the specific surface area significantly decreased,because these silica gels contain e.g. alkali metals at a highconcentration and are likely to undergo change in the structure orparticle shape.

Test on Crushing Strength of Silica Gel

1.4±0.2 g of each of the silica gels of Examples 1 and ComparativeExample 2 was packed in a tablet machine for IR (tablet diameter 20 mm),and a pressure of 4.0 ton/cm² was applied thereto at room temperaturefor 3 minutes. Each powder was taken out from the tablet machine, andthe specific surface area and the pore volume were measured by anitrogen adsorption and desorption method, and the results are shown inTable 3.

TABLE 2 Before test on heat stability After test on heat in water (m²/g)stability in water (m²/g) Example 1 859 57 Example 2 706 46 Example 3409 56 Comparative 855 28 Example 2 Comparative 599 4 Example 3Comparative 413 3 Example 4

TABLE 3 Specific surface area before Pore volume before and and aftertest on crushing after test on crushing (m²/g) (cm³/g) Before test Aftertest Before test After test Example 1 819 791 0.74 0.62 Comparative 762572 0.53 0.32 Example 2

A high mechanical strength is required for a silica gel obtainedindustrially. In a case where it is used as it is, particles may breakdue to contact among particles or with an apparatus to form fine powder,thus causing a problem in operation of equipment in which the silica gelis involved. Further, in a case where it is used as a molded product, itis important that it is resistant to the pressure applied duringmolding, and one having no adequate strength will be formed into amolded product wherein the pore characteristics of the original silicagel can not be put to a good use. A conventional silica gel has noadequate strength. As evident from Table 3, the silica gel of thepresent invention has an adequate crushing strength.

The novel silica gel of the present invention is excellent in heatresistance and hydrothermal resistance, is highly stable and has a highpurity as compared with a conventional silica gel. Further, a silica gelhaving desired physical properties can be produced by a relativelysimple method using a silicon alkoxide as a material.

The silica gel of the present invention can be used for conventionalapplications of silica gel, and particularly when it is used as e.g. acatalyst carrier or a membrane reactor, deterioration in performancetends to be small, and it can be used more stably for a long period oftime.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

This application is based on Japanese patent applications JP 2000-197558and JP 2001-066167 filed in the Japanese Patent Office on Jun. 30, 2000and Mar. 9, 2001 the entire contents of each are hereby incorporated byreference.

1. A method of producing a silica gel, which comprises: hydrolyzing asilicon alkoxide, thereby forming a hydrogel; and subjecting thehydrogel to a hydrothermal treatment substantially without aging thehydrogel, thereby producing said silica gel having the followingcharacteristics: (a) a pore volume ranging from 0 6 to 1.6 ml/g, (b) aspecific surface area ranging from 300 to 900 m²/g, (c) a mode diameter(Dmax) of pores of less than 20 nm, (d) a volume of pores havingdiameters within ±20% of Dmax of at least 50% of the total pore volume;(e) it is amorphous, and (f) the content of metal impurities is at most500 ppm.
 2. The method for producing a silica gel according to claim 1,wherein a hydrogel having a breaking stress of at most 6 MPa issubjected to the hydrothermal treatment.
 3. The method for producing asilica gel according to claim 1, wherein the hydrothermal treatment iscarried out at a temperature ranging from 50 to 150° C. for from 1 to 10hours.
 4. The method for producing a silica gel according to claim 1,wherein an ammonia water is used for the hydrothermal treatment isconducted in ammonia water.
 5. The method for producing a silica gelaccording to claim 1, wherein hydrolysis of said silicon alkoxide isconducted in the absence of a template.
 6. A silica gel, produced by aprocess comprising: hydrolyzing a silicon alkoxide, thereby forming ahydrogel; and subjecting the hydrogel to a hydrothermal treatmentsubstantially without aging the hydrogel, thereby producing said silicagel having the following characteristic: (a) a pore volume ranging from0.6 to 1.6 ml/g, (b) a specific surface area ranging from 300 to 900m²/g, (c) a mode diameter (Dmax) of pores of less than 20 nm, (d) avolume of pores having diameters within ±20% of Dmax of at least 50% ofthe total pore volume; (e) it is amorphous, and (f) the content of metalimpurities is at most 500 ppm.
 7. The silica gel according to claim 6,wherein a hydrogel having a breaking stress of at most 6 MPa issubjected to the hydrothermal treatment.
 8. The silica gel according toclaim 6, wherein the hydrothermal treatment is carried out at atemperature ranging from 50 to 150° C. for from 1 to 10 hours.
 9. Thesilica gel according to claim 6, wherein the hydrothermal treatment isconducted in ammonia water.
 10. The silica gel according to claim 6,wherein the pore volume ranges from 0.8 to 1.6 ml/g.
 11. The silica gelaccording to claim 6, wherein the specific surface area ranges from 400to 900 m²/g.
 12. The silica gel according to claim 6, wherein the modediameter (Dmax) is at least 2 nm.
 13. The silica gel according to claim6, wherein the volume of pores having diameters within ±20% of Dmax isat least 60% of the total pore volume.
 14. The silica gel according toclaim 6, wherein the content of metal impurities is at most 10 ppm. 15.The silica gel according to claim 14, wherein the content of metalimpurities is at most 1 ppm.
 16. The silica gel according to claim 6,wherein the differential pore volume at the mode diameter (Dmax) rangesfrom 5.0 to 12.0 ml/g.
 17. The silica gel according to claim 6, whereinthe value of Q4/Q3 in solid state Si-NMR is at least 1.3.
 18. The silicagel according to claim 6, wherein hydrolysis of said silicon alkoxide isconducted in the absence of a template.
 19. The silica gel according toclaim 6, wherein hydrolysis of said silicon alkoxide is conducted in theabsence of a template.
 20. A silica gel which has the followingcharacteristics: (a) a pore volume ranging from 0.6 to 1.6 ml/g, (b) aspecific surface area ranging from 300 to 900 m²/g, (c) a mode diameter(Dmax) of pores of less than 20 nm, (d) a volume of pores havingdiameters within ±20% of Dmax of at least 50% of the total pore volume;(e) it is amorphous, and (f) the content of metal impurities is at most500 ppm.
 21. The silica gel according to claim 20, wherein the porevolume ranges from 0.8 to 1.6 ml/g.
 22. The silica gel according toclaim 20, wherein the specific surface area ranges from 400 to 900 m²/g.23. The silica gel according to claim 20, wherein the mode diameter(Dmax) is at least 2 nm. 24.The silica gel according to claim 20,wherein the volume of pores having diameters within ±20% of Dmax is atleast 60% of the total pore volume.
 25. The silica gel according toclaim 20, wherein the content of metal impurities is at most 10 ppm. 26.The silica gel according to claim 25, wherein the content of metalimpurities is at most 1 ppm.
 27. The silica gel according to claim 20,wherein the differential pore volume at the mode diameter (Dmax) rangesfrom 5.0 to 12.0 ml/g.
 28. The silica gel according to claim 20, whereinth value of Q4/Q3 in solid state Si-NMR is at least 1.3.
 29. The silicagel according to claim 20, which is produced by means of a step ofhydrolyzing a silicon alkoxide.
 30. A method of producing a silica gel,which comprises: hydrolyzing a silicon alkoxide, thereby forming ahydrogel which has breaking stress of at most 6 MPa; and subjecting thehydrogel to a hydrothermal treatment substantially without aging thehydrogel, thereby producing said silica gel having the followingcharacteristics: (a) a pore volume ranging from 0.6 to 1.6 ml/g, (b) aspecific surface area ranging from 300 to 900 m²/g, (c) a mode diameter(Dmax) of pores of less than 20 nm, (d) a volume of pores havingdiameters within ±20% of Dmax of at least 50% of the total pore volume;(e) it is amorphous, and (f) the content of metal impurities is at most500 ppm.